|Publication number||US7043713 B2|
|Application number||US 10/640,171|
|Publication date||May 9, 2006|
|Filing date||Aug 12, 2003|
|Priority date||Feb 25, 2000|
|Also published as||US6769109, US20020010903, US20040049759|
|Publication number||10640171, 640171, US 7043713 B2, US 7043713B2, US-B2-7043713, US7043713 B2, US7043713B2|
|Inventors||Robert Osann, Jr., Shafy Eltoukhy, Shridhar Mukund, Lyle Smith|
|Original Assignee||Lightspeed Semiconductor Corp.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (35), Non-Patent Citations (6), Referenced by (5), Classifications (10), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation of application Ser. No. 09/877,470, filed Jun. 8, 2001, now U.S. Pat. No. 6,769,109 which is a continuation-in-part of application Ser. No. 09/512,783, filed Feb. 25, 2000, now U.S. Pat. No. 6,694,491 and claims priority to Provisional Application No. 60/231,059, filed Sep. 8, 2000, both incorporated by reference herein.
This invention generally relates to Application Specific Integrated Circuits (ASICs), and particularly to Programmable Logic Arrays (PLAs) incorporated into non-reprogrammable ASIC devices.
Application Specific Integrated Circuits (ASICs) have become widespread in the semiconductor industry. ASICs are generally integrated circuits that are customizable to implement a circuit specified by a design engineer or other user (a “user-defined” or “user-specified” circuit).
ASICs can be divided into two groups: (1) those that are “multi-time configurable” or “reconfigurable” (i.e., they can be programmed with data and reprogrammed) and (2) those that are only “one-time configurable” or “non-reconfigurable.” Multi-time configurable devices include Programmable Logic Devices (PLDs) and FPGAs. One-time configurable devices include some gate arrays and module-based arrays (MBAs). A general discussion of each technology will be useful.
Multi-Time Configurable Devices
Generally, most PLDs are based on a PLA structure where a function is expressed in product terms and sum terms to be implemented. Each product term is generated by a gate that can be programmed to form the AND of any subset of the inputs and their complements. Subsets of the product terms can be summed in a set of programmable OR gates.
PLAs are constructed in the form of arrays, with the input lines being orthogonal to the product lines, as shown in the generalized circuit diagram of
Product-terms are often implemented with a wired-OR mechanism, where multiple programmable transistors 116 are connected to the product-term line 110 and a pull-up 118 is used, as shown in the generalized circuit diagram of
A second type of PLA structure (sometimes referred to as a “Full PLA”) is shown in
Flexibility of PLA structures, including those of
Each of the conventional PLA structures described in
One-Time Configurable Devices
The second category of ASIC mentioned is a one-time configurable ASIC, frequently gate arrays, MBAs, or standard cells. Typically, these one-time configurable devices are configured (or customized) by “mask-programming”—i.e., these devices are customized once using various mask and etch steps to form interconnections dictated by a user-defined circuit. Once configured, these mask-programmed devices are not reconfigurable.
An example MBA is shown in the generalized block diagram of
Gate arrays are known in the art and are similar in many respects to MBAs, except they are frequently composed of a “sea of gates”—prefabricated transistors that are (for the most part) unconnected to one another. These gate arrays are also configured using mask and etch techniques to interconnect the active devices and thereby form a user-defined circuit.
More specifically, as shown in
Another type of mask-programmed device is a “standard cell.” A standard cell is similar to an MBA, but instead of predefined function blocks, it includes custom cells that are optimized for performing a respective designated function. In other words, compared to MBA function blocks, the custom cells of a standard cell device have adjusted transistor size and placement and have eliminated extraneous devices. Thus, standard cells are customized in all the layers shown in
Once the customizing metal layers have been designed and implemented, the mask-programmed device (e.g., MBA or standard cell) is said to have been configured—but it cannot be reconfigured. As a result, designers of mask-programmed ASICs typically implement high-speed logic functions, including state machines and control logic, with conventional non-reconfigurable ASIC gates. Still, in many instances, when a mask-programmed device is configured, many parts of the implemented circuit are not fully verified. Moreover, during the development process, changes often need to occur in certain parts of the circuit, such as the control logic. Therefore, frequently portions of the circuit that are to be reconfigurable or may need to be changed are typically separately implemented in a separate PLD device. Thus, it is desirable to maintain some level of reconfigurability in at least part of the circuit, thereby minimizing the use of multiple ICs.
In accordance with the invention, a method for customizing a one-time configurable integrated circuit to include a multi-time configurable structure is disclosed. Such a method includes, in one embodiment, receiving a description of circuit functionality from a user for implementation in the one-time configurable device, where the functionality includes a portion that is designated by the user to be reconfigurable. A method in accordance with an embodiment of the invention then models a reconfigurable structure that has enough capacity to accommodate the designated functionality. Optionally, some embodiments of the invention add in more capacity than is required to implement the designated functionality to allow for future reprogramming. The method then embeds the reconfigurable structure in the one-time configurable device. In certain embodiments, the one-time configurable device can be a mask-programmed MBA, gate array, or standard cell, while the reconfigurable structure is a PLA or modified PLA.
The ability to reconfigure a portion of a one-time configurable device is especially useful where time-to-market and flexibility are particularly important. Such reconfigurability is also useful for high-risk circuits and state machine control. For instance, if a designer using a mask-programmed device is unsure of part of his/her design, the designer may want the ability to reconfigure a portion of that design without having to resort to multiple chips. In particular, because most circuit changes in a user-defined circuit occur in the control logic, a PLA structure incorporated into a mask-programmed device in accordance with the invention is particularly useful for implementing a circuit's control logic.
The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:
In accordance with the invention, a multi-time configurable structure such as a reconfigurable PLA structure is incorporated into a one-time configurable ASIC. In accordance with the invention, the following generalized steps can be performed to obtain reconfigurability. First, the designer (user) defines the planned functionality for the ASIC as a whole, including the functionality that the designer believes will be required for the particular reconfigurable logic block, such as a state machine. The designer designates that portion of the functionality that is to be reconfigurable, and software next takes the specified description of the designated functionality to model a PLA structure suitable for implementation of that reconfigurable logic. The designer and/or software then adds a selected number of AND terms, OR terms, nonregistered outputs, registered outputs, state-bits, etc., to allow for expansion or modification of the functionality in the event of later reprogramming. This modeled PLA structure (with added capacity) is then incorporated into the rest of the ASIC design.
Hence, as shown in the generalized PLA structure 900 of
Creating a PLA Within an ASIC
The particular steps used to form a reprogrammable structure with a non-reprogrammable device in accordance with an embodiment of the invention are shown with respect to
Next, in some embodiments of the invention, additional capacity 1108 is added to the PLA format file 1106 (to accommodate changes and/or modifications in functionality later) in step 1110 to generate a structural netlist 1112 for the PLA. The amount of additional capacity 1108 can be determined by the circuit designer or it can be added automatically by software (e.g., by automatically making the PLA 10% larger than required to accommodate the user's functionality).
In still other embodiments a PLA in accordance with an embodiment of the invention could even be designed to perform two or more diverse tasks. In other words, a single PLA could be built, but it may be used in circuitry that could be applied to two (or more) distinct applications. In such a case, the functionality for the PLA for each task will be different and the PLA will be later programmed based on one set of functionality or the other. Thus, in accordance with an embodiment of the invention, the initial size of the PLA will be generated according to the larger set of functionality and then additional capacity can be added (if desired). In another embodiment of the invention, the size of the PLA will be generated to accommodate the union of the set of designs being merged, essentially creating an array that accommodates a superset of the functionality.
In one embodiment of the invention, structural netlist generation 1110 creates netlist 1112 in terms of an ASIC library. Nonetheless, it is to be understood that in a preferred embodiment of the invention, although an ASIC cell library is utilized, the entire PLA is a custom PLA—i.e., the ASIC cell library does not include a primitive logic function for a PLA having a designated number of inputs, outputs, p-terms, etc., although other embodiments may utilize such a primitive logic function in a library. The structural netlist 1112 models the PLA that is actually to be constructed.
After the netlist 1112 has been generated, PLA physical layout and timing extraction is performed 1114. As a result, a PLA placement 1116 is generated, defining a physical relationship among the functions that form the PLA. Such a placement can be used in many embodiments of the invention to optimize PLA performance.
In the meantime, a structural netlist 1118 for the rest of the ASIC has been synthesized 1120 from the HDL file 1102. The PLA placement 1116 is combined with a structural netlist for the complete ASIC (minus the PLA) 1118 to perform the physical layout and timing extraction 1122 for the entire ASIC device. Once the layout for the entire device has been performed, the ASIC can be fabricated (e.g., completely fabricated or selected layers mask-programmed) 1124, resulting in custom ASIC 1126 that includes a custom and reprogrammable PLA.
Simulation of the ASIC can be performed at various stages during the ASIC creation process of
Many of the steps shown in
As should be further understood from the flow of
Generating a Programming Pattern
The structure for a PLA device in accordance with the invention will vary with the ASIC technology utilized. For instance, in one embodiment using a standard cell where all the layers are customized, the structure of a PLA device in accordance with the invention can be designed to look much like conventional PLAs as described with respect to FIG. 2—e.g., product terms implemented with a wired-OR mechanism by connecting multiple transistors to a product term line. Likewise, with many gate arrays where transistors in function blocks are not interconnected (or are minimally connected) in each function block, a
This wired-OR approach, however, typically requires a PLA structure with a regular array, i.e., one that has wired-OR lines that have the same length and loading, because it usually uses sense amps coupled to that line to amplify signals. If the wired-OR lines are not all the same length with the same loading, then the sense amps would necessarily also have to be different, creating more complicated design issues.
Therefore, in other embodiments, the structures will be different. In MBA devices, for example, circuits in each function block are pre-defined (i.e., designed and/or formed prior to receiving the user-defined circuit) and often do not contain leftover or otherwise available transistors to implement conventional PLA configurations. The existing functionality and/or devices must be adapted.
An example of a function block 1200 in an MBA used in one embodiment of the invention is functionally shown in
Bit-storage unit 1270 receives as an input the output from dual-control multiplexer 1260 on line 1264. In one embodiment of the invention, bit-storage unit 1270 is a pair of cross-coupled inverters 1272 and 1274 as shown in
Dual-control multiplexer 1280 receives as a first input the output of bit-storage unit 1270 on line 1276. The other input to multiplexer 1280 is coupled to signal DA on line 1278, an input into function block 1200. Similar to multiplexer 1260, multiplexer 1280 has two signal paths, each controlled by a respective switch 1281 or 1283. Signal S on line 1282 controls switch 1281 while signal L on line 1284 controls switch 1283.
Bit-storage unit 1288 receives as an input the output of dual-control multiplexer 1280 on line 1286. Like bit-storage unit 1270, bit-storage unit 1288 is, in one embodiment, composed of a pair of cross-coupled inverters 1290 and 1292, where inverter 1292 is weaker than inverter 1272, and where inverter 1292 is selectively enabled by signal L on line 1284.
Multiplexer 1296 receives as a first input the output of bit-storage unit 1288 on line 1294. The second input to multiplexer 1296 is received from the output 1276 of bit-storage unit 1270. Multiplexer 1296 further has a select input SX, which multiplexer 1296 receives on line 1297 and which selects one of the multiplexer's inputs to be output onto line 1298.
Line 1298 is coupled to inverter 1301, which serves as a buffering mechanism and which outputs signal Q on line 1302.
In addition, function block 1200 also includes select and enable logic, which selects the various switches in multiplexers 1260 and 1280 as well as enables inverters 1274 and 1292 in bit-storage units 1270 and 1288, respectively. The select and enable logic in one embodiment includes NOR gate 1304, NAND gate 1310, NAND gate 1316, and inverters 1322 and 1324.
NOR gate 1304 has a first input MC on line 1306, an input into function block 1200, and a second input received from the output of NAND gate 1310 via line 1268. NOR gate 1304 outputs signal M on line 1266, which controls switch 1261 and enables inverter 1274.
Inputs to NAND gate 1310 are EN on line 1312 and AS on line 1314, both inputs to function block 1200. The output 1268 from NAND gate 1310 is the signal F which controls switch 1263 in multiplexer 1260.
NAND gate 1316 receives as inputs signal SC on line 1318 and signal S2 on line 1320, both inputs to function block 1200. The output of NAND gate 1316 is coupled to inverter 1324, which outputs signal S on line 1282 to control switch 1281 in multiplexer 1280.
Inverter 1322 also receives signal S2 on line 1320 and outputs signal L on line 1284 to control switch 1283 of multiplexer 1280 as well as inverter 1292 in bit-storage unit 1288.
While select and enable logic for function block 1200 is shown in
In some embodiments of the invention, function block 1200 forms only a portion of a larger function block. A more detailed explanation of these function blocks and MBAs can be found in Function Block Architecture for Gate Array, Ser. No. 08/821,475, filed on Mar. 21, 1997, and Test Circuitry for ASICs, Ser. No. 08/985,790, filed on Dec. 5, 1997, both incorporated by reference herein.
A second embodiment of the invention utilizes a function block such as that shown in
In addition in
The function blocks 1200 and 1300 described with respect to
In particular, a function block 1200/1300 can be configured to function as a latch or a flip-flop. For instance, to implement a flip-flop, the data input (D-input) of the flip-flop is applied to input DS 1262. The output Q 1302 from the function block is also the output (Q) of the flip-flop. A clock signal (CLK) is applied to inputs MC 1306 and SC 1318. An enable signal is applied to input EN 1312. A clear signal or a preset signal is applied to inputs AS 1314 and S2 1320. DA 1278 is connected to a logical low for a clear signal or to a logical high for a preset signal. SX 1297 is tied to a logical low. In this manner, a data bit input at DS will pass serially through bit storage unit 1270 and bit storage unit 1288, and the implementation acts as a master-slave configuration. Thus, when input data is input on line DS and switch 1261 closes (driven by CLK) then the input data will be stored in units 1270 and 1288. When switch 1261 opens, bit storage units 1270 and 1288 remain undisturbed and hold the last bit stored. A latch can be implemented in a similar manner, but only one bit storage unit needs to be utilized.
Referring now to
Each latch 1404, 1406 is programmed to store a value that dictates how the core cell 1402 will behave upon receiving an input signal on line 1410 into the PLA. Programming is done in accordance with Table 1:
Thus, when latch 1404 stores a logical 1 and latch 1406 stores a logical 0, the signal input on line 1410 will be output on line 1412. When latch 1404 stores a logical 0 and latch 1406 stores a logical 1, the signal input on line 1410 will be inverted on line 1412. When both latches 1404 and 1406 store a logical 1, it indicates a “don't care” condition—no matter what is input on line 1410, the product-term will not be affected. But if latches 1404 and 1406 both store a logical 0, the entire p-term will be disabled (a 0 input into an AND gate always results in a 0 output).
An alternative embodiment of a PLA core cell 1402 is shown in
A comparison of
Nonetheless, both the embodiments illustrated in
Further, wired-OR structures are notorious for static power consumption since anytime the product-term line is pulled low, current flows. But since the structures of
A PLA using a structure such as those shown in
Because of their ease of use and rapid production time, an MBA may be used by circuit designers in designing prototypes. Once a design has been finalized, however, a user may wish to optimize the circuit implemented by an MBA. Such optimization may take place using a “standard cell” or a “partial standard cell.” As used herein, the term “standard cell” generally refers to ASICs having a higher degree of customizability than an MBA, even if such ASIC is not completely customizable, usually having a substantial number of masks that are customized for an individual design. For instance, compared to the MBA, transistor size and placement within the function blocks may be altered and/or unused transistors may be eliminated. Routing may also be optimized for the particular application. Thus, in some embodiments of the invention a circuit first implemented in an MBA may be converted to one implemented in a standard cell. Still in many embodiments, the PLA in a standard cell retains user programmability and uses a programming structure similar to that of the MBA (e.g.,
Programming (Loading the Programming Pattern)
In order to program and/or reprogram a PLA embedded in a one-time configurable ASIC in accordance with the present invention, several alternatives exist. In one embodiment, a JTAG (or similar) controller can be used as will be understood in the art to shift a programming pattern through a test scan path.
A second embodiment uses an array structure with function blocks such as those of
Referring back to
Now referring again to
To load the flip-flops and thus program the PLA, all rows and columns are first selected to be in a program mode by applying logical high values to all of the CR lines 1716 j and/or all of the CC lines 1712 i. PLA program values are placed in a shift register 1702, where the output of each stage 1704 i of the shift register 1702 is coupled to the TD input of the first flip-flop 1701 i0 in each column, in one embodiment of the invention. Applying TCLK 1734, PLA program values are applied one at a time, from shift register stages 1704 i to the respective daisy-chains where they are simultaneously shifted through each column 1720 i via the daisy-chains. When all values have been shifted to the appropriate flip-flops, CC and CR are de-asserted.
In many embodiments of the invention, it will be desirable to have the ability to load a default programming pattern into the “programming memory” (e.g., latches or flip-flops) of the PLA. Such a default pattern would be written into the programming memory by a signal that is, for instance, related to the system power-on-reset or similar signal, setting all programming memory elements to a predetermined value.
For instance, shown in
In another embodiment, such as implementing a PLA in a standard cell, or even loading the PLA dynamically during the operation, the loading mechanism will be able to choose between a default pattern (if one is used) and a new pattern used for reprogramming. In such a case, the circuit of
The embodiment shown in
It should be understood that the particular embodiments described above are only illustrative of the principles of the present invention, and various modifications could be made by those skilled in the art without departing from the scope and spirit of the invention. Thus, the scope of the present invention is limited only by the claims that follow.
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|U.S. Classification||716/117, 257/E27.105, 716/121|
|International Classification||G06F17/50, H01L27/118, H03K19/177|
|Cooperative Classification||H01L27/118, H03K19/17708|
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